key_buffer_size

This page is original PmaControl documentation. It keeps a practical index structure for MySQL optimization work, without copying third-party content or depending on an external product.

Role

MyISAM index cache.

Diagnosis

SHOW VARIABLES LIKE 'key_buffer_size';
SHOW GLOBAL STATUS LIKE '%key%';

The status counters do not always use the same name as the variable. Use this command as a starting point, then complete with performance_schema, SHOW ENGINE INNODB STATUS, or system metrics.

Tuning

Low importance on an InnoDB platform. Do not reserve too much RAM for a rarely used engine.

Risks

• A global value that is too high can increase memory consumed at startup.
• A per-connection value can explode with max_connections.
• A change must be tested with a before/after measurement and rollback.